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USUB8

Unsigned parallel byte-wise subtraction.

Syntax

USUB8{cond} {Rd}, Rn, Rm

where:

cond

is an optional condition code.

Rd

is the destination general-purpose register.

Rm, Rn

are the general-purpose registers holding the operands.

Operation

This instruction subtracts each byte of the second operand from the corresponding byte of the first operand and writes the results into the corresponding bytes of the destination. The results are modulo 28. It sets the APSR GE flags.

Register restrictions

You cannot use PC for any operand.

You can use SP in A32 instructions but this is deprecated. You cannot use SP in T32 instructions.

GE flags

This instruction does not affect the N, Z, C, V, or Q flags.

It sets the GE flags in the APSR as follows:

GE[0]

for bits[7:0] of the result.

GE[1]

for bits[15:8] of the result.

GE[2]

for bits[23:16] of the result.

GE[3]

for bits[31:24] of the result.

It sets a GE flag to 1 to indicate that the corresponding result is greater than or equal to zero, meaning a borrow did not occur. This is equivalent to a SUBS instruction setting the C condition flag to 1.

You can use these flags to control a following SEL instruction.

Availability

This instruction is available in A32 and T32.

There is no 16-bit version of this instruction in T32.

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